We have shown for the first time in human breast cancer patients that residual tumors after standard chemotherapy are enriched for a CD44+/CD24-/low population of breast cancer stem or tumor-initiating cells (TIC) that exhibit enhanced mammosphere-forming efficiency (MSFE) and display an increase in xenograft transplant outgrowths in mice (1, 2, 11). In addition, we have determined from gene expression analysis and shRNA library screening of the CD44+/CD24-/low-mammosphere-forming population that the top canonical pathways involved in breast TIC self-renewal include Stat3 signaling (3). Using virtual ligand screening, we identified three small-molecule probes that target the Src homology (SH) 2 domain of Stat3 and that competitively and selectively inhibited Stat3 activation (5). The most active of these hits, C188, was shown to dramatically reduce human-into-mouse breast cancer tumorgraft volumes and to increase tumor-free survival of engrafted mice 4-fold when used in combination with standard chemotherapy (3). We performed 2-D similarity screening using the scaffold of C188 as the query structure and a commercial compound library (Life Chemicals) followed by 3-D pharmacophore analysis and identified 33 active derivatives. One derivative (C188-9), now owned by StemMed, is 1-log more active than C188 at inhibiting Stat3 binding in SPR assays and in blocking ligand-mediated Stat3 phosphorylation (6). In addition, C188-9 exhibited minimal toxicity in mice up to a dose of 100 mg/kg/day for 14 days, did not inhibit normal murine bone marrow colony formation in vitro, and, prevented weight loss and sarcopenia in a murine model of cachexia. The specific hypothesis that will be examined in this Phase I STTR is that C188-9 or one of its derivatives will be highly active against Stat3, will be well tolerated in mice, and will demonstrate efficacy in breast tumorgraft models. We have proposed 2 tightly focused specific aims to pursue this hypothesis: AIM 1: To synthesize candidate third (3rd) generation C188 probes based on modifications of the C188-9 lead structure and to identify those with increased potency and favorable ADMET features. Synthesis of new probes is straightforward; probes with greater potency and favorable ADMET features will be identified using well-established assays. AIM 2. To examine the most potent and least toxic 3rd generation Stat3 probes for the ability to target breast TICs and inhibi tumorgraft growth when used in combination with standard chemotherapy. Two C188-9 derivatives that are the most potent, least toxic, and have the best ADMET profile will be examined for the ability to target TIC and inhibit tumor growth when combined with standard chemotherapy using the most informative subset of the 29 breast tumorgraft models we have developed. These studies will set the stage for a Phase II STTR application in which we will propose GMP synthesis and in vivo ADMET studies of the most promising lead compound identified in preparation for a Phase I clinical trial in metastatic breast cancer.